1. Field of Invention
This invention relates to a capacitance type electromagnetic flowmeter for detecting, via electrostatic capacitances, signal voltages corresponding to flow rates produced in a fluid undergoing measurement and, more particularly, to an improved capacitance type electromagnetic flowmeter which detects signals from a closed flow tube without being affected by the flow velocity distribution in a fluid when the flow tube is completely filled with the fluid, and which also detects such signals without being affected by either changes in the level of the fluid or flow velocity distribution even when the closed flow tube is not completely filled with the liquid. Furthermore, the invention relates to an improved capacitance type electromagnetic flowmeter capable of detecting the signal voltages without being affected by either changes in the level of the fluid being measured or flow velocity distribution even in an open conduit having a free liquid surface.
2. Description of the Prior Art
FIG. 1 shows a conventional electromagnetic flowmeter designed to reduce the effect of flow velocity distribution, such as disclosed, for example, in Japanese Patent Publication No. 28325/1980 entitled "Electromagnetic Flowmeter".
In such conventional device, a tube 10 is provided which has an electrically insulated inner surface. Magnets 11 and 12 are disposed on opposite sides of tube 10 to apply magnetic flux .PHI. to tube 10. A plurality of pairs of electrodes 13a,13b,14a, 14b, 15a, 15b, 16a,16b,and 17a, 17b, are mounted on the insulated inner surface of tube 10. Electrodes 13a and 13b are arranged in diametrically opposite relation to each other in the center of tube 10. Electrodes 14a and 14b are respectively located a given distance above electrodes 13a and 13b, and electrodes 15a and 15b are respectively located above electrodes 14a and 14b and spaced a given distance from electrodes 14a and 14b. Electrodes 16a,17a, 16b, and 17b are successively regularly located downward from electrodes 13a and 13b, respectively. Electrodes 13a and 13b are connected to a first amplifier 18. Electrodes 14a and 14b are connected to a second amplifier 19. Electrodes 15a and 15b are connected to a third amplifier 20. Electrodes 16a and 16b are connected to a fourth amplifier 21. Electrodes 17a and 17b are connected to a fifth amplifier 22.
The outputs from amplifiers 18-22 are applied to a converter 23, which applies the average of the outputs across output terminals 24a and 24b. Amplifiers 18-22, converter 23, and other components together form a signal processor 25.
In the FIG. 1 device, numerous electrode pairs are arranged radially around tube 10. Consequently, the output signal appearing across the output terminals 24a and 24b indicates the average of the flow velocities of fluid flowing across various portions of tube 10.
Accordingly, if the flow velocity distribution is not symmetrical with respect to the axis, the output voltage developed across output terminals 24a and 24b is in proportion to the flow rate of the fluid flowing through tube 10.
The conventional electromagnetic flowmeter described above cannot sufficiently suppress the effect of the flow velocity distribution unless numerous electrodes are provided in contact with the liquid. This makes the structure and arrangement of the electrodes complex. In addition, liquid might leak from the electrodes. If a small diameter tube is used, there is no space for accommodating the electrodes. Thus, in practice it is difficult to fabricate this type of flowmeter. Also, the signal processor must incorporate numerous amplifiers for averaging out the signal voltages. This increases costs.
A further problem encountered by the prior art is that the electrodes are widely spaced from each other. In particular, when the inside of the tube is not completely filled with liquid or when the liquid level varies, such as in the case of of an open conduit, the detected signal voltage takes the form of a discrete signal. This limits the accuracy of calculation of the flow rates.
Another problem arises when the tube is not completely filled. In that case, the mutually spaced electrodes may be brought into and out of contact with the fluid being measured by variations in the free liquid surface. Thus, electrochemical potentials on the surfaces of the electrodes will vary greatly, thereby resulting in large variations of indicated values.